A lovely tribute to those involved and the objects of their various studies. The initial presentation of the extra letters raised a grin. Thank you, Vismut.
9a - "drill" is extra word.and the definition is "throw up" and if you put an S at each end of your answer you'll get a word that, while normally meaning a lazy person, in this instance means "(mud)flats"
PS: If you're wondering why the word "mud" is missing, Chambers gives one definition of the the plural "flats" as meaning "an are of land covered by shallow water".
Given the severe constraints put on the rest of the grid by the thematic material, I thought it was quite a feat to produce a symmetric grid that only contained a few obscure words.
Yes an excellent puzzle. I always learn something from Vismut. I liked her extra words, 8 letter phrase and having to "unjumble" instead of the usual jumble.